1. Field of the Invention
The present invention relates generally to semiconductor circuit structures and, more specifically to a structure that reduces the low frequency oscillations created by the injection of carriers into the semiconductor structure.
2. Description of the Prior Art
Low frequency oscillations are produced when a charge is injected into a semi-insulating substrate. The low frequency oscillations, when viewed with a spectrum analyzer, appear as single or multiple relatively broad peaks or as broad noise in a frequency range of a few hertz (Hz) to a few kilohertz (KHz). In high dynamic range or high gain circuits, these low frequency oscillations are mixed and amplified and appear as increased noise at the output of a circuit, degrading the overall performance of the semiconductor system. The increased noise can manifest itself as anomalous phase noise, spurious sidebands, or peaks superimposed on the expected 1/f noise. All of these manifestations may critically limit the performance of the circuits and create a significant source of noise in monolithic microwave integrated circuits (MMICs).
The mechanism believed to be responsible for the low frequency oscillations is the field-enhanced capture of carriers by deep level traps. As illustrated in the prior art structure of FIG. 1, the trapping of carriers 16 with increasing electric field produces a negative differential resistance and the formation of a high field domain traveling from an electron injecting contact 18 towards a second contact 22. Low frequency oscillations 24 are produced within the high field domain between the contacts 18 and 22. The frequency of the low frequency oscillations 24 is determined by the transit time of the high field domain between the two contacts (18, 22). The threshold, frequency and amplitude of the low frequency oscillations typically depend on substrate type, substrate surface polishing, electric field voltage applied between contacts or devices, distance between contacts (carrier diffusion length), substrate temperature, and carrier density injected into a substrate. Conventional semiconductor device systems attempt to reduce the low frequency oscillation effect in several ways.
The publication xe2x80x9cLow Frequency Oscillations In GaAs IC""sxe2x80x9d, by Miller et al., IEEE Tech. Digest GaAs IC Symposium, pp. 31-34, 1985 discloses reducing low-frequency oscillation effect by using several techniques that decrease or eliminate the deep level traps that capture and release low-frequency oscillation producing electrons. One such technique describes changing the integrated circuit design. This technique includes designing a circuit that minimizes the electrical field near critical devices or contacts pads and changing the layout of a circuit to reduce the coupling of oscillating leakage currents to active devices and contact pads. This technique may reduce the low frequency oscillations but does not guarantee the complete suppression of the oscillations. Additionally, this technique is circuit dependent and does not represent a general solution.
In the Miller publication, alternate methods for reducing or eliminating deep level traps to minimize low frequency oscillations are also proposed. Specifically, one method suggests choosing substrate materials such as highly chromium (Cr) doped LEC and horizontal Bridgeman gallium arsenide (GaAs) that have a reduced tendency for low frequency oscillation. A variation on this method, proposed by Miller et al., is to grow a thick (approximately 10 xcexcm) OM-VPE GaAs buffer on the substrate before growing the active device structure.
Finally, in conventional systems, the substrate wafer surface may be selectively cleaned prior to growing the active device epilayers in an attempt to reduce low frequency oscillation effects. However, the results derived from selective cleaning processes are unpredictable and are not comprehensive.
Based on techniques known in the art for semiconductor device systems, a semiconductor device structure that reduces the effects of low frequency oscillations resulting from carrier injection is highly desirable.
The present invention provides an integrated circuit structure that includes a substrate wafer having an active device layer disposed on a surface of the substrate wafer and having an electrically conductive element contained therein. The integrated circuit structure further comprises a barrier disposed between the substrate wafer and the active device layer, where the barrier blocks carriers injected into the substrate wafer and reduces low frequency oscillation effect.